The present invention relates to a lead frame and a method for producing a lead frame. More particularly, the present invention relates to a lead frame capable of being produced by an etching process and a method for producing such a lead frame.
Referring to FIG. 8 showing the configuration of a generally known resin-sealed semiconductor package 60, such as a plastic lead frame package, when fabricating the resin-sealed semiconductor package 60, a semiconductor device 69 is mounted on a lead frame 61 of 42 ALLOY (a Ni/Fe alloy comprising 42% Ni:), and then the semiconductor device 69 is sealed with a resin 65. The number of the inner leads 63 of the lead frame 61 is equal to that of the bonding pads 66 of the semiconductor device 69. The lead frame 61 has a die pad 62 to be mounted with the semiconductor device 69, and outer leads 64 to be electrically connected to the associated circuits. The tips of the inner leads 63 are connected electrically to the bonding pads 66 of the semiconductor device 69 with bonding wires 67, respectively. After mounting the semiconductor device 69 on the die pad 62 and connecting the bonding pads 66 to the inner leads 63, respectively, the semiconductor device 69 is sealed with the resin 65 to protect the semiconductor device 69 from external stresses and contaminants.
Recently, there has been growing demand for the miniaturization and reduction in thickness of resin-sealed semiconductor packages employing lead frames like the lead frame 61 (plastic lead frame packages) and the increase of the number of terminals of resin-sealed semiconductor packages as electronic apparatuses are miniaturized progressively and the degree of integration of semiconductor devices increases progressively. Consequently, recent resin-sealed semiconductor packages, particularly, quad flat packages (QFPs) and thin quad flat packages (TQFPs) have each a greatly increased number of pins.
Lead frames having fine inner leads arranged at small pitches among lead frames for semiconductor packages are fabricated by a photolithographic etching process, while lead frames having inner leads arranged at comparatively large pitches among lead frames for semiconductor packages are fabricated by press working. However, lead frames having a large number of fine inner leads to be used for forming semiconductor packages having a large number of pins are fabricated by subjecting a blank of a thickness on the order of 0.25 mm to an etching process regardless of size.
The etching process for forming a lead frame having fine inner leads will be described hereinafter with reference to FIG. 7. First a copper alloy or 42 alloy thin sheet 51 of a thickness on the order of 0.25 mm (blank for a lead frame) is cleaned perfectly (FIG. 7(a)). Then, a photoresist, such as a water-soluble casein photoresist containing potassium dichromate as sensitive agent, is spread in photoresist films 52 over the opposite major surfaces of the thin sheet 51 as shown in FIG. 7(b). Then, the photoresist films 52 are exposed through a mask of a predetermined pattern to light emitted by a high-pressure mercury lamp, the thin sheet 51 is immersed in a developer for development to form a patterned photoresist films 53 as shown in FIG. 7(c). Then, the thin sheet 51 is subjected, when need be, to a hardening process, a washing process and such, and then an etchant containing ferric chloride as a principal component is sprayed against the thin sheet 51 to etch through portions of the thin sheet 51 not coated with the patterned photoresist films 53 so that inner leads 63 of predetermined sizes and predetermined shapes are formed as shown in FIG. 7(d). Then, the patterned resist films 53 are removed, the patterned thin sheet 51 is washed to complete a lead frame 61 having the inner leads 63 of desired shapes as shown in FIG. 7(e).
Predetermined areas of the lead frame 61 thus formed by the etching process are silver-plated. After being washed and dried, an adhesive polyimide tape is stuck to the inner leads 63 for fixation, predetermined tab bars are bent, when need be, and the die pad is depressed.
In the etching process, the etchant etches the thin sheet 51 in both the direction of the thickness and directions perpendicular to the direction of the thickness, which limits the miniaturization of inner lead pitches of lead frames.
Since the thin sheet 51 is etched from both the major surfaces as best shown in FIG. 7(d) during the etching process, it is said, when the lead frame has a line-and-space shape, that the smallest possible intervals between the lines are in the range of 50 to 100% of the thickness of the thin sheet 51.
From the viewpoint of forming the outer leads 64 having a sufficient strength, generally, the thickness of the thin sheet 51 must be about 0.125 mm or above.
Furthermore, the width of the inner leads 63 must be in the range of 70 to 80 .mu.m for successful wire bonding. When the etching process as illustrated in FIG. 7 is employed in fabricating a lead frame, a thin sheet of a small thickness in the range of 0.125 to 0.15 mm is used and inner leads are formed by etching so that the fine tips thereof are arranged at a pitch of about 0.165 mm.
However, recent miniature resin-sealed semiconductor packages requires inner leads arranged at pitches in the range of 0.13 to 0.15 mm, far smaller than 0.165 mm. When a lead frame is fabricated by processing a thin sheet of a reduced thickness, the strength of the outer leads of such a lead frame is not large enough to withstand external forces that may be applied thereto in the following processes including an assembling process and a chip mounting process. Accordingly, there is a limit to the reduction of the thickness-of the thin sheet to enable the fabrication of a minute lead frame having fine leads arranged at very small pitches by etching.
An etching method previously proposed to overcome such difficulties subjects a thin sheet to an etching process to form a lead frame after reducing the thickness of portions of the thin sheet corresponding to the inner leads of the lead frame by half etching or pressing to form the fine inner leads by etching without reducing the strength of the outer leads. However, problems arise in accuracy in the following processes when the lead frame is formed by etching after reducing the thickness of the portions corresponding to the inner leads by pressing; for example, the smoothness of the surfaces of plated areas is unsatisfactory, the inner leads cannot be formed in a flatness and a dimensional accuracy required to clamp the lead frame accurately for bonding and molding, and a platemaking process must be repeated twice making the lead fabricating process intricate. It is also necessary to repeat a platemaking process twice when the thickness of the portions of the thin sheet corresponding to the inner leads is reduced by half etching before subjecting the thin sheet to an etching process for forming the lead frame, which also makes the lead frame fabricating process intricate. Thus, this previously proposed etching method has not yet been applied to practical lead frame fabricating processes.
In some cases, oxides and hydroxides are formed on the surfaces of portions of a lead frame and the surfaces of the portions are tarnished due to etching when the lead frame is formed by etching a thin sheet of a copper alloy, i.e., a blank for forming the lead frame, which is inferred to be due to the action of dissolved oxygen and the like contained in a washing liquid (water) used for cleaning the lead frame after etching on the highly reactive metal surfaces exposed by etching. Such an oxide and a hydroxide are hard to dissolve and cannot be completely removed by a chemical polishing process, i.e., a preprocessing process preceding a plating process, to which the lead frame formed by etching is subjected before a plating process.
The tips of the inner leads of the lead frame to be electrically connected to the terminals of a semiconductor device by bonding wires are plated with gold or the like. If the surfaces of areas to be plated of the tips of the inner leads are coated with the oxide or the hydroxide, the oxide or the hydroxide cannot be completely removed by the chemical polishing process, consequently, the areas of the tips of the inner lead cannot be uniformly plated and faulty wire-bonding results.
Accordingly, there has been a demand for a lead frame having fine inner leads arranged at very small pitches suitable for forming a miniaturized semiconductor package having an increased number of pins, having outer leads having a sufficiently high strength, and having less portions tarnished by an oxide or a hydroxide, and for a method of fabricating such a lead frame.